The present invention relates to a switching apparatus having an actuator in which when an object comes into contact with the actuator, a switch output indicative of the detection of the object is generated in response to a displacement of the actuator.
Such a kind of switching apparatus is used to detect an object and control various kinds of machines or apparatuses. For example, in the production line in an unmanned factory, such a switching apparatus is used to detect when an object such as a product or the like to be worked has been transported to a predetermined position, so that a working machine is automatically actuated.
Hitherto, a limit switch has generally been used as such a kind of switching apparatus. A contact mechanism provided in the limit switch is switched between the on (conductive) state and the off (nonconductive) state in association with the displacement of the actuator which is caused by the contact with an object or in response to the actuator being returned to the original position by an urging means such as a return spring or the like.
In the switching apparatus with the foregoing constitution, a defective contact or the like occurs due to the following causes. Namely, a displacement of the actuator does not accurately occur or the actuator is not properly returned to the original position because of an increase in frictional force; water enters the switching apparatus due to a defective sealing, causing the internal circuits or contacts to be short-circuited; contacts are fused and bonded or bent or damaged; chips or dusts are interposed between contacts; and the like. Thus, a malfunction such that the object cannot be correctly detected occurs. When the object cannot be correctly detected, even if the object has been carried to a predetermined position, the machine or apparatus is not actuated, so that a defective machining of a product occurs or an unfinished product is erroneously taken out as a normal product. Hitherto, only when such an unexpected accident occurred, the defective operation of the limit switch is found out. Consequently, the production line must be stopped to repair the limit switch. There is a problem such that even if the production line is temporarily stopped as mentioned above, adverse influences are also exerted on the other producing processes and the producing efficiency deteriorates.
Therefore, in order to enable the limit switch to certainly perform the on/off operations, there is an apparatus having a mechanism to prevent a contact from becoming fused and bonded, a contact peeling-off mechanism to forcedly turn off the contact even if the contact was fused and bonded, or the like. However, this apparatus cannot cope with the case where the actuator is not properly returned because of an increase in frictional force when the apparatus is used, the case where chips, dusts or the like are interposed between contacts of the limit switch, so that the limit switch is not switched from the off state to the on state, or the like.
There is also a system in which a life time of the limit switch is stored in an external control unit including a computer and the life time is managed. In this system, the operating time of the production line is calculated and the cumulative operating time is compared with the life time stored, and when the cumulative operating time exceeds the life time, a failure prediction signal is output. However, there is a drawback such that the external control unit increases in size and becomes expensive. Moreover, since a failure cannot be accurately predicted in accordance with the actual situation, there are inconveniences such that an abnormality occurs before the failure prediction signal is output, and in spite of the fact that the apparatus is normally operating, a failure prediction signal is output, and the like.